Dynamic routing protocols, such as the Enhanced Interior Gateway Routing Protocol (EIGRP), Intermediate System-to-Intermediate System (IS-IS), Open Shortest Path First (OSPF), and Routing Information Protocol (RIP) Version 2, and static routes support variable-length subnet masks (VLSMs). VLSM enables an organization to use more than one subnet mask within the same network address space. VLSM allows you to conserve IP addresses and efficiently use the available address space. Implementing VLSM is often referred to as “subnetting a subnet.”

Note

You may want to carefully consider the use of VLSMs. It is easy to make mistakes during address assignments and difficult to monitor networks that use VLSMs. The best way to implement VLSMs is to keep your existing addressing plan in place and gradually migrate some networks to VLSMs to recover address space.

The following example uses two different subnet masks for the class B network address of 172.16.0.0. A subnet mask of /24 is used for LAN interfaces. The /24 mask allows 256 subnets with 254 host IP addresses on each subnet. The final subnet of the range of possible subnets using a /24 subnet mask (172.16.255.0) is reserved for use on point-to-point interfaces and assigned a longer mask of /30. The use of a /30 mask on 172.16.255.0 creates 64 subnets (172.16.255.0–72.16.255.252) with 2 host addresses on each subnet.

Note	To ensure unambiguous routing, you must not assign 172.16.255.0/24 to a LAN interface in your network.

Router(config)# interface Ethernet 0/0
Router(config-if)# ip address 172.16.1.1 255.255.255.0
Router(config-if)# exit
Router(config)# interface Serial 0/0
Router(config-if)# ip address 172.16.255.5 255.255.255.252
Router(config-if)# exit
Router(config)# router rip 
Router(config-router)# network 172.16.0.0

Static routes are user-defined routes that cause packets moving between a source and a destination to take a specified path. Static routes can be important if the router cannot build a route to a particular destination. They are also useful in specifying a gateway of last resort to which all unroutable packets will be sent.

To configure a static route, use the ip route command in global configuration mode.

Static routes remain in the router configuration until you remove them (by using the no form of the ip route command). However, you can override static routes with dynamic routing information through the assignment of administrative distance values. An administrative distance is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers.

Each dynamic routing protocol has a default administrative distance, as listed in the table below. For a configured static route to be overridden, the administrative distance of the static route should be higher than that of the dynamic routing protocol.

Static routes that point to an interface are advertised through dynamic routing protocols, regardless of whether redistribute static router configuration commands were specified for those routing protocols. Static routes that point to an interface are advertised because the routing table considers these routes as connected routes and hence, these routes lose their static nature. However, if you define a static route to an interface that is not connected to one of the networks defined by a network command, no dynamic routing protocol will advertise the route unless a redistribute static command is specified for the protocols.

When an interface goes down, all static routes associated with that interface are removed from the IP routing table. Also, when the software can no longer find a valid next hop for the address specified as the address of the forwarding router in a static route, the static route is removed from the IP routing table.

Default routes, also known as gateways of last resort, are used to route packets that are addressed to networks not explicitly listed in the routing table. A device might not be able to determine routes to all networks. To provide complete routing capability, network administrators use some devices as smart devices and give the remaining devices default routes to the smart device. (Smart devices have routing table information for the entire internetwork.) Default routes can be either passed along dynamically or configured manually into individual devices.

Most dynamic interior routing protocols include a mechanism for causing a smart device to generate dynamic default information, which is then passed along to other devices.

You can use the ip default-gateway global configuration command to define a default gateway when IP routing is disabled on a device. For instance, if a device is a host, you can use this command to define a default gateway for the device. You can also use this command to transfer a Cisco software image to a device when the device is in boot mode. In boot mode, IP routing is not enabled on the device.

Unlike the ip default-gateway command, the ip default-network command can be used when IP routing is enabled on a device. When you specify a network by using the ip default-network command, the device considers routes to that network for installation as the gateway of last resort on the device.

Gateways of last resort configured by using the ip default-network command are propagated differently depending on which routing protocol is propagating the default route. For Interior Gateway Routing Protocol (IGRP) and Enhanced Interior Gateway Routing Protocol (EIGRP) to propagate the default route, the network specified by the ip default-network command must be known to IGRP or EIGRP. The network must be an IGRP- or EIGRP-derived network in the routing table, or the static route used to generate the route to the network must be redistributed into IGRP or EIGRP or advertised into these protocols by using the network command. The Routing Information Protocol (RIP) advertises a route to network 0.0.0.0 if a gateway of last resort is configured by using the ip default-network command. The network specified in the ip default-network command need not be explicitly advertised under RIP.

Creating a static route to network 0.0.0.0 0.0.0.0 by using the ip route 0.0.0.0 0.0.0.0 command is another way to set the gateway of last resort on a device. As with the ip default-network command, using the static route to 0.0.0.0 is not dependent on any routing protocols. However, IP routing must be enabled on the device. IGRP does not recognize a route to network 0.0.0.0. Therefore, it cannot propagate default routes created by using the ip route 0.0.0.0 0.0.0.0 command. Use the ip default-network command to have IGRP propagate a default route.

EIGRP propagates a route to network 0.0.0.0, but the static route must be redistributed into the routing protocol.

Depending on your release of the Cisco software, the default route created by using the ip route 0.0.0.0 0.0.0.0 command is automatically advertised by RIP devices. In some releases, RIP does not advertise the default route if the route is not learned via RIP. You might have to redistribute the route into RIP by using the redistribute command.

Default routes created using the ip route 0.0.0.0 0.0.0.0 command are not propagated by Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS). Additionally, these default routes cannot be redistributed into OSPF or IS-IS by using the redistribute command. Use the default-information originate command to generate a default route into an OSPF or IS-IS routing domain.

When a network address is subnetted, the first subnet obtained is called subnet zero.

Consider a Class B address 172.16.0.0. By default, a Class B address has 16 bits reserved for representing the host portion, thus allowing 65534 (2¹⁶–2) valid host addresses. If network 172.16.0.0/16 is subnetted by borrowing three bits from the host portion, eight (2³) subnets are obtained. The table below shows the subnets obtained by subnetting the address 172.16.0.0, the resulting subnet mask, the corresponding broadcast address, and the range of valid host addresses.

In the above table, the first subnet (subnet 172.16.0.0) is called subnet zero.

The class of the network subnetted and the number of subnets obtained after subnetting have no role in determining subnet zero. It is the first subnet obtained when subnetting the network address. Also, when you write the binary equivalent of the subnet zero address, all the subnet bits (bits 17, 18, and 19 in this case) are zeros. Subnet zero is also known as the all-zeros subnet.

When a network address is subnetted, the last subnet obtained is called the all-ones subnet.

With reference to the above table, the last subnet obtained when you subnet network 172.16.0.0 (subnet 172.16.224.0/19) is called the all-ones subnet.

The class of the network subnetted and the number of subnets obtained after subnetting have no role in determining the all-ones subnet. Also, when you write the binary equivalent of the all-ones subnet, all the subnet bits (bits 17, 18, and 19 in this case) are ones, hence the name.

By default, most IP routing protocols install a maximum of four parallel paths in a routing table. Static routes always install six paths. The exception is BGP, which by default allows only one path (the best path) to the destination. However, BGP can be configured to use equal and unequal cost multipath load sharing. See the "BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS-VPN" feature in the BGP Configuration Guide for more information.

The number of parallel paths that you can configure to be installed in the routing table is dependent on the installed version of the Cisco IOS software. To change the maximum number of parallel paths allowed, use the maximum-paths command in router configuration mode.

Multi-interface load splitting allows you to efficiently control traffic that travels across multiple interfaces to the same destination. The traffic-share min router configuration command specifies that if multiple paths are available to the same destination, only paths with the minimum metric will be installed in the routing table. The number of paths allowed is never more than six. For dynamic routing protocols, the number of paths is controlled by the maximum-paths router configuration command. The static route source can install six paths. If more paths are available, the extra paths are discarded. If some installed paths are removed from the routing table, pending routes are added automatically.

You can configure the Cisco IOS software to redistribute information from one routing protocol to another. For example, you can configure a device to readvertise EIGRP-derived routes using RIP or to readvertise static routes using EIGRP. Redistribution from one routing protocol to another can be configured in all IP-based routing protocols.

You can also conditionally control the redistribution of routes between routing domains by configuring route maps between two domains. A route map is a route filter that is configured with permit and deny statements, match and set clauses, and sequence numbers. To define a route map for redistribution, use the route-map command in global configuration mode.

The metrics of one routing protocol do not necessarily translate into the metrics of another. For example, the RIP metric is hop count and the EIGRP metric is a combination of five metric values. In such situations, a dynamic metric is assigned to the redistributed route. Redistribution in these cases should be applied consistently and carefully in conjunction with inbound filtering to avoid the creation of routing loops.

The following examples illustrate the use of redistribution with and without route maps. The following example shows how to redistribute all OSPF routes into EIGRP:

Router(config)# router eigrp 1 
Router(config-router)# redistribute ospf 101 
Router(config-router)# exit 

The following example shows how to redistribute RIP routes, with a hop count equal to 1, into OSPF. These routes will be redistributed into OSPF as external LSAs with a metric of 5, metric-type of type 1, and a tag equal to 1.

Router(config)# router ospf 1 
Router(config-router)# redistribute rip route-map rip-to-ospf 
Router(config-router)# exit 
Router(config)# route-map rip-to-ospf permit 
Router(config-route-map)# match metric 1 
Router(config-route-map)# set metric 5 
Router(config-route-map)# set metric-type type 1 
Router(config-route-map)# set tag 1 
Router(config-route-map)# exit 

The following example shows how to redistribute OSPF learned routes with tag 7 as a RIP metric of 15:

Router(config)# router rip 
Router(config-router)# redistribute ospf 1 route-map 5 
Router(config-router)# exit 
Router(config)# route-map 5 permit 
Router(config-route-map)# match tag 7 
Router(config-route-map)# set metric 15 

The following example shows how to redistribute OSPF intra-area and inter-area routes with next-hop routers on serial interface 0/0 into BGP with a metric of 5:

Router(config)# router bgp 50000 
Router(config-router)# redistribute ospf 1 route-map 10 
Router(config-router)# exit 
Router(config)# route-map 10 permit 
Router(config-route-map)# match route-type internal 
Router(config-route-map)# match interface serial 0 
Router(config-route-map)# set metric 5 

The following example redistributes two types of routes into the integrated IS-IS routing table (supporting both IP and CLNS). The first type is OSPF external IP routes with tag 5; these routes are inserted into Level 2 IS-IS link-state packets (LSPs) with a metric of 5. The second type is ISO-IGRP-derived CLNS prefix routes that match CLNS access list 2000; these routes are redistributed into IS-IS as Level 2 LSPs with a metric of 30.

Router(config)# router isis 
Router(config-router)# redistribute ospf 1 route-map 2 
Router(config-router)# redistribute iso-igrp nsfnet route-map 3 
Router(config-router)# exit 
Router(config)# route-map 2 permit 
Router(config-route-map)# match route-type external 
Router(config-route-map)# match tag 5 
Router(config-route-map)# set metric 5 
Router(config-route-map)# set level level-2 
Router(config-route-map)# exit 
Router(config)# route-map 3 permit 
Router(config-route-map)# match address 2000 
Router(config-route-map)# set metric 30 
Router(config-route-map)# exit 

In the following example, OSPF external routes with tags 1, 2, 3, and 5 are redistributed into RIP with metrics of 1, 1, 5, and 5, respectively. The OSPF routes with a tag of 4 are not redistributed.

Router(config)# router rip 
Router(config-router)# redistribute ospf 101 route-map 1 
Router(config-router)# exit 
Router(config)# route-map 1 permit 
Router(config-route-map)# match tag 1 2 
Router(config-route-map)# set metric 1 
Router(config-route-map)# exit 
Router(config)# route-map 1 permit 
Router(config-route-map)# match tag 3 
Router(config-route-map)# set metric 5 
Router(config-route-map)# exit 
Router(config)# route-map 1 deny 
Router(config-route-map)# match tag 4 
Router(config-route-map)# exit 
Router(config)# route map 1 permit 
Router(config-route-map)# match tag 5 
Router(config-route-map)# set metric 5 
Router(config-route-map)# exit 

The following example shows how a route map is referenced by using the default-information router configuration command. Such referencing is called conditional default origination. OSPF will generate the default route (network 0.0.0.0) with a type 2 metric of 5 if 172.16.0.0 is in the routing table.

Router(config)# route-map ospf-default permit 
Router(config-route-map)# match ip address 1 
Router(config-route-map)# set metric 5 
Router(config-route-map)# set metric-type type-2 
Router(config-route-map)# exit 
Router(config)# access-list 1 172.16.0.0 0.0.255.255 
Router(config)# router ospf 101 
Router(config-router)# default-information originate route-map ospf-default 

The Default Passive Interfaces feature simplifies the configuration of distribution devices by allowing all interfaces to be set as passive by default. In ISPs and large enterprise networks, many distribution devices have more than 200 interfaces. Obtaining routing information from these interfaces requires configuration of the routing protocol on all interfaces and manual configuration of the passive-interface command on interfaces where adjacencies were not desired.

Filtering sources of routing information prioritizes routing information gathered from different sources because some pieces of routing information may be more accurate than others. An administrative distance is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers. Numerically, an administrative distance is an integer from 0 to 255. In general, the higher the value the lower the trust rating. An administrative distance of 255 means that the routing information source cannot be trusted at all and should be ignored.

In a large network, some routing protocols and some routers can be more reliable than others as sources of routing information. Also, when multiple routing processes are running on the same router for IP, the same route may be advertised by more than one routing process. By specifying administrative distance values, you enable a router to intelligently discriminate between sources of routing information. The router will always pick the route whose routing protocol has the lowest administrative distance.

There are no guidelines for assigning administrative distances because each network has its own requirements. You must determine a reasonable matrix of administrative distances for a network as a whole.

Note	You can use the administrative distance to rate the routing information from routers that are running the same routing protocol. However, using the administrative distance for this purpose can result in inconsistent routing information and forwarding loops.

In the following example, the router eigrp global configuration command configures EIGRP routing in autonomous system 1. The network command specifies EIGRP routing on networks 192.0.2.16 and 172.16.0.0. The first distance router configuration command sets the default administrative distance to 255, which instructs the router to ignore all routing updates from routers for which an explicit distance has not been set. The second distance command sets the administrative distance to 80 for internal EIGRP routes and to 100 for external EIGRP routes. The third distance command sets the administrative distance to 120 for the router with the address 172.16.1.3.

Router(config)# router eigrp 1 
Router(config-router)# network 192.0.2.16 
Router(config-router)# network 172.16.0.0 
Router(config-router)# distance 255 
Router(config-router)# distance eigrp 80 100 
Router(config-router)# distance 120 172.16.1.3 0.0.0.0 

Note	The distance eigrp command must be used to set the administrative distance for EIGRP-derived routes.

The following example assigns the router with the address 192.0.2.1 an administrative distance of 100 and all other routers on subnet 192.0.2.0 an administrative distance of 200:

Router(config-router)# distance 100 192.0.2.1 0.0.0.0 
Router(config-router)# distance 200 192.0.2.0 0.0.0.255 

However, if you reverse the order of these two commands, all routers on subnet 192.0.2.0 are assigned an administrative distance of 200, including the router at address 192.0.2.1:

Router(config-router)# distance 200 192.0.2.0 0.0.0.255 
Router(config-router)# distance 100 192.0.2.1 0.0.0.0 

Note	Administrative distances should be applied carefully and consistently to avoid the creation of routing loops or other network failures.

In the following example, the administrative distance value for learned IP routes is 90. Preference is given to these IP routes rather than routes with the default administrative distance value of 110.

Router(config)# router isis
Router(config-router)# distance 90 ip
 

Policy-based routing (PBR) is a more flexible mechanism than destination routing for routing packets. It is a process whereby a router puts packets through a route map before routing them. The route map determines which packets are routed to which router next. You can enable PBR if you want certain packets to be routed some way other than the obvious shortest path. Possible applications for policy-based routing include protocol-sensitive routing, source-sensitive routing, routing based on interactive versus batch traffic, and routing based on dedicated links.

To enable PBR, you must identify the route map to be used for PBR and create the route map. The route map specifies the match criteria and the resulting action if all match clauses are met.

A packet arriving on a specified interface will be subject to PBR, except when its destination IP address is the same as the IP address of the router’s interface. To disable fast switching of all packets arriving on this interface, use the ip policy route-map command in interface configuration mode.

To define the route map to be used for PBR, use the route-map command in global configuration mode.

To define the criteria by which packets are examined to learn if they will follow PBR, use either the match length command or the match ip address command or both in route map configuration mode. The match length command allows you to configure policy routing based on the Level 3 length of the packet, and the match ip address command allows you to policy route packets based on the criteria that can be matched with an extended access list.

The following example provides two sources with equal access to two different service providers. Packets that arrive on asynchronous interface 1 from the source 10.1.1.1 are sent to the router at 172.16.6.6 if the router has no explicit route for the destination of the packets. Packets that arrive from the source 172.17.2.2 are sent to the router at 192.168.7.7 if the router has no explicit route for the destination of the packets. All other packets for which the router has no explicit route to the destination are discarded.

Router(config)# access-list 1 permit ip 10.1.1.1 
Router(config)# access-list 2 permit ip 172.17.2.2 
Router(config)# interface async 1 
Router(config-if)# ip policy route-map equal-access 
Router(config-if)# exit 
Router(config)# route-map equal-access permit 10 
Router(config-route-map)# match ip address 1 
Router(config-route-map)# set ip default next-hop 172.16.6.6 
Router(config-route-map)# exit 
Router(config)# route-map equal-access permit 20 
Router(config-route-map)# match ip address 2 
Router(config-route-map)# set ip default next-hop 192.168.7.7 
Router(config-route-map)# exit 
Router(config)# route-map equal-access permit 30 
Router(config-route-map)# set default interface null 0 
Router(config-route-map)# exit
 

You can set IP header precedence bits in the router when PBR is enabled. The precedence setting in the IP header determines how packets are treated during times of high traffic. When packets containing these headers arrive at another router, the packets are ordered for transmission according to the precedence set if the queuing feature is enabled. The router does not honor the precedence bits if queuing is not enabled, and the packets are sent in FIFO order. You can change the precedence setting by using either a number or a name.

The table below lists the possible IP Precedence values (numbers and their corresponding names), from the least important to the most important.

The QoS Policy Propagation via BGP feature allows you to classify packets by IP precedence based on BGP community lists, BGP autonomous system paths, and access lists. After a packet has been classified, you can use other Quality of Service features such as committed access rate (CAR) and Weighted Random Early Detection (WRED) to specify and enforce policies to fit your business model.

Before you configure policy propagation via BGP, perform the following basic tasks:

• Configure BGP and Cisco Express Forwarding or distributed Cisco Express Forwarding. To configure BGP, refer to the BGP Configuration Guide. To configure Cisco Express Forwarding and distributed Cisco Express Forwarding, refer to the Cisco Express Forwarding Configuration Guide.

• Define a policy.

• Apply the policy through BGP.

• Configure the BGP community list, BGP autonomous system path, or an access list and enable the policy on an interface.

• Enable CAR or WRED to use the policy. To enable CAR, see the chapter “Configuring Committed Access Rate” in the Quality of Service Solutions Configuration Guide. To configure WRED, see the chapter “Configuring Weighted Random Early Detection” in the Quality of Service Solutions Configuration Guide.

Note

Before the QoS Policy Propagation via BGP feature can work, you must enable BGP and Cisco Express Forwarding or distributed Cisco Express Forwarding on the router. Subinterfaces on ATM interfaces that have the bgp-policy command enabled must use Cisco Express Forwarding because distributed Cisco Express Forwarding is not supported. Distributed Cisco Express Forwarding uses the Versatile Interface Processor (VIP) rather than the Route Switch Processor (RSP) to perform forwarding functions.

Key management is a method of controlling the authentication keys used by routing protocols. Not all protocols support key management. Authentication keys are available for Director Response Protocol (DRP) Agent, Enhanced Interior Gateway Routing Protocol (EIGRP), and Routing Information Protocol (RIP) Version 2.

You can manage authentication keys by defining key chains, identifying the keys that belong to the key chain, and specifying how long each key is valid. Each key has its own key identifier (specified using the key chain configuration command), which is stored locally. The combination of the key identifier and the interface associated with the message uniquely identifies the authentication algorithm and the message digest algorithm 5 (MD5) authentication key in use.

You can configure multiple keys with lifetimes. Only one authentication packet is sent, regardless of how many valid keys exist. The software examines the key numbers in ascending order and uses the first valid key it encounters. The lifetimes allow for overlap during key changes.

You can redistribute routes from one routing domain into another, with or without controlling the redistribution with a route map. To control which routes are redistributed, configure a route map and reference the route map from the redistribute command.

The tasks in this section describe how to define the conditions for redistributing routes (a route map), how to redistribute routes, and how to remove options for redistributing routes, depending on the protocol being used.

To filter routing protocol information, perform the tasks in this section.

Note	When routes are redistributed between OSPF processes, no OSPF metric is preserved.